One goal in forming many high aspect ratio (HAR) contacts in integrated circuits is the deposition of a very thin layer of titanium at the bottom of an etched opening, such as a via or trench. This layer facilitates electrical contact between the contact material which will subsequently fill the opening and the material to which it connects, e.g. doped polysilicon. Prior to this deposition, an etching solution is utilized to form the openings necessary in an insulative layer, for example, Boro-Phospho-Silicate Glass (BPSG) or other insulator layer to provide the HAR contact access to an exposed underlying conductive material. This conductive material is usually formed of doped polysilicon, or some other conductive substance. The thin titanium layer reacts with this polysilicon, usually in the presence of heat, to form a titanium silicide layer.
Unfortunately, the materials used for etching the BPSG layer typically leave a polymer residue which ends up coating the inside and especially the bottom of the HAR contact opening. This residue must be removed before a titanium deposition can be initiated. The traditional way of removing this residue has been through the utilization of an oxygen (O2) plasma strip step. The etch polymer residue reacts with the oxygen and is removed.
This process however, can leave behind a silicon rich oxide residue layer at the bottom of the opening because the oxygen also reacts with the polysilicon. The silicon rich layer can be represented as SiOx, with x being between 0 and 2. This layer can also contain impurities, for example, carbon and fluorine impurities. This silicon rich oxide layer residue is substantially non-conductive, and therefore interferes with the deposition and of a conductive titanium material in the etched opening and the subsequent formation of a desirable titanium silicide layer at the bottom of the contact opening. This in turn affects the conductive performance of a conductor formed in the opening.
Traditional “wet chemistries” have been employed to remove the silicon rich oxide layer formed as a result of O2 plasma stripping to further assist in preparing the HAR bottom surface opening for Ti deposition. However, these methodologies have not been entirely successful in removing the silicon rich oxide layer. These chemical methods are not generally selective to the silicon rich oxide layer, and thus they can undesirably also etch the BPSG sidewalls of the opening and thereby increase its size.
What is therefore needed is an improved process for removing polymer etch residue from HAR contact openings which eliminates the formation of a silicon rich oxide layer within the contact opening. The process should also effectively eliminate the etch polymer from both the sides and bottom of the HAR opening without undesirably increasing its size.